a. Characteristics. There are over 2,000 species of bacteria. Bacteria are
relatively large in size when compared to viruses. Some large bacteria are 100 microns
in length, while many common bacteria are from 5 to 10 microns (5,000 to 10,000
millimicrons) long. Thus, while bacteria are much larger than viruses, a microscope
must still be used to observe them. Bacteria are usually unicellular (one-celled). They
reproduce by binary fission (a process in which one bacterium divides to form two
bacteria). This process of binary fission occurs quickly. In fact, many bacteria
reproduce in this way once every 30 minutes. Under ideal conditions, one bacterium
can produce at least 4.64 x 1021 bacteria within 24 hours.
b. Classification of Bacteria. Bacteria are most commonly classified by their
shape and stain characteristics.
(1) Shape. There are four basic bacterial shapes: cocci (round), bacilli
(rod-shaped), spirilla (curved rod), and spirochete (corkscrew). See figure 1-1.
Figure 1-1. The shapes of bacteria